1. Field of the Invention
The present invention relates to a polarization independent optical isolator and an optical transmitting/receiving apparatus, and more particularly, to an LD/PD (laser diode/photo diode) integrated optical device in an optical module that is used in a communication system which transmits and receives a signal through a single line bi-directionally.
2. Description of the Related Art
In recent years, a PON (passive optical network) system has been widely employed in an optical fiber access network. The PON system is a communication network in which an optical signal from an OLT (optical line terminal) serving as a base station side device is split by a star coupler, and then shared by a plurality of subscriber side devices ONU (optical network unit). A down signal from the OLT is used with a wavelength of 1.55 μm band, and an up signal from the ONU is used with a wavelength of 1.3 μm band.
As an optical module for the PON, there is used an LD/PD integrated optical device (BIDI (bi-directional) optical transmitting and receiving module) as shown in FIG. 2. In the BIDI optical transmitting and receiving module, the LD that constitutes a transmitter section and the PD that constitutes a receiver section are generally received within a CAN type package. Also, transmitter light and receiver light are divided by a wavelength division element that is called “WDM (wavelength division multiplexing) filter” by using a difference between the wavelength of the transmitter light and the wavelength of the receiver light.
In order to expand the capacity of transmission in the future, there has been studied a WDM-PON system in which an optical signal of one wavelength is allocated to each of subscribes by means of a technique of multiplexing optical signals that are different in the wavelength, and transmitting the optical signals through one optical fiber. In the WDM-PON, the LD/PD integrated optical device (BIDI optical transmitting and receiving module) in the case where the bi-directional communication is conducted with the same wavelength becomes essential in the future because the wavelengths of the optical signals that are transmitted or received are identical with each other.
In the WDM-PON, the transmitter light and the receiver light are identical in the wavelength with each other. For that reason, it is impossible to apply a wavelength division element (WDM filter) shown in FIG. 2. Therefore, the WDM filter is replaced by the polarization independent optical isolator.
As shown in FIG. 3, the polarization independent optical isolator includes first and second birefringent plates 1 and 4, a Faraday element (Faraday rotator) 2, and a λ/2 plate 3. Referring to FIG. 3, transmitter light from the LD is linearly polarized in a y-direction. Also, arbitrary linearly polarized light is assumed as receiver light that is received by the PD, and has both of a y-component and an x-component. In addition, a magnetic field is applied to the Faraday element 2 in a z-direction.
FIG. 4 is a diagram showing light polarization states when the light has passed through the respective elements of the optical isolator. Referring to FIG. 3, the light from the LD (transmitter light: indicated by the solid arrow in FIG. 3) is rotated by 45 degrees with respect to a light traveling direction by means of the λ/2 plate 3 (FIG. 4[B]) after having passed through the second birefringent plate 4 (FIG. 4[A]). Then, the polarization direction of the transmitter light is rotated by 45 degrees with respect to a direction of the magnetic field by means of the Faraday element 2 (FIG. 4[C]). As a result, the polarization direction returns to a state of the light when the light is outputted from the LD. After that, the transmitter light passes through the first birefringent plate 1 and is coupled into the optical fiber.
On the other hand, light from the optical fiber side (receiver light: indicated by the dashed arrow in FIG. 3) is divided into normal light and abnormal light which are perpendicular in the polarization direction to each other (orthogonal: refer to FIG. 4[D]) by means of the first birefringent plate 1, the normal light (upper side of FIG. 3) goes straightly and is outputted, and the abnormal light (lower side of FIG. 3) is outputted from a position that is translated by a given distance d from an output point of the normal light. The Faraday element 2 rotates the polarization directions of the normal light and the abnormal light by 45 degrees with respect to the direction of the magnetic field (FIG. 4[E]). After that, the receiver light (the normal light and the abnormal light) is rotated by 45 degrees with respect to the light traveling direction when passing through the λ/2 plate 3 (FIG. 4[F]). After that, the normal light and the abnormal light are polarized and combined together by the second birefringent plate 4, and then received by the PD.
The transmitter light and the receiver light can be divided from each other by means of the above-described polarization independent optical isolator in principle. However, in an actual optical module, the LD and the PD are received in the individual CAN type packages, respectively. Herein, when those two CAN type packages are disposed in parallel, those packages must be so disposed as to be out of contact with each other. As a result, in the case where those CAN type packages are disposed in parallel, it is necessary to ensure a longer distance than the sum of the radius of the CAN type package (LD package) of the LD and the radius of the CAN type package (PD package) of the PD as a distance d1 (FIG. 3) between the optical axes of the receiver light and the transmitter light. Then, it is necessary to prepare an optical isolator that divides the receiver light into the normal light and the abnormal light by a light beam distance d corresponding to the distance d1.
On the contrary, up to now, as in Prior Art 1 shown in FIG. 5, the first birefringent plate 1 and the second birefringent plate 4 are thickened to extend the distance d, to thereby ensure the light beam distance d1. Alternatively, as in Prior Art 2 shown in FIG. 6, the LD package and the PD package are so disposed as to be orthogonal to each other, and a reflecting mirror 5 that bends an optical path at a right angle is disposed between the LD package and the second birefringent plate 4, to thereby prevent the distance d from being extended.
FIG. 7 shows a structural example of an optical device (BIDI) to which Prior Art 2 is applied. As shown in FIG. 7, the respective elements 1 to 5 are adhered to each other so as to shorten the length in the optical path direction. In this case, when the respective lengths of the first birefringent plate 1, the Faraday element 2, the λ/2 plate 3, the second birefringent plate 4, and the reflecting mirror 5 in the optical path direction (horizontal length in FIG. 7) are set to, for example, 1.25 mm, 3.3 mm, 0.2 mm, 1.25 mm, and 0.2 mm or lower, the light beam distance between the transmitter light and the receiver light can be set to 0.125 mm.
Also, as the prior art documents related to the present invention, there are the inventions disclosed in the following documents.
[Patent document 1] JP 5-341229 A
[Patent document 2] JP 7-253559 A
[Patent document 3] JP 9-18422 A
However, the method of Prior Art 1 causes a problem in that the optical device is upsized as much as the increased thicknesses of the first birefringent plate 1 and the second birefringent plate 4. On the other hand, in the method of Prior Art 2, as shown in FIG. 7, the PD package and the LD package must be so disposed as to be orthogonal to each other so that the PD package and the LD package come out of contact with each other. FIG. 7 exemplifies the approximate sizes of the LD package and the PD package. In the example shown in FIG. 7, the width length (diameter) of a light transmitting portion (CAN portion) in the LD package is 4.2 mm, and the width length (diameter) of a light receiving portion (CAN portion) in the PD package is 4.0 mm. As shown in FIG. 7, even in the case where the PD package and the LD package are disposed to be orthogonal to each other so that those packages come as close as possible to each other in a non-contact state, a distance must be provided in consideration of the radius of the LD package between the reflecting mirror 5 and the PD package. In the example shown in FIG. 7, it is necessary to provide a distance of about 2 mm between the center of the reflecting mirror 5 and the PD package surface. Thus, in Prior Art 2, there arises a problem in that the optical device cannot be reduced in the size for the prevention of a physical interference (contact) between the LD package and the PD package.